1. Field of the Invention
This invention relates to infinite network polymers for coating and cladding optical fibers, specifically, to infinite network polymers containing fluorine for coating and cladding optical fiber.
2. Description of the Prior Art
Organic polymer coatings on optical fibers improve protection of the silica glass surface from abrasive damage and environmental effects, such as corrosion due to stress and moisture permeation. In addition, they act as a buffer in reducing the sensitivity of the optical fiber to microbending loss.
Small external forces due to an irregular surface of the optical fiber are sufficient to cause lateral deformation, mode coupling and optical loss. Polymer coatings can be designed to reduce static fatigue and microbending loss. The elastic or relaxation modulus of the polymer is a good indicator of how effective the coating is in protecting the clad fiber from these effects. It has been observed that coatings with relaxation modulus values less than 10.sup.8 dyn/cm.sup.2 are effective in reducing microbending loss.
Stress-induced corrosion by water is associated with stress concentrations at surface flaws under tensile deformation. The only ways to protect the glass fiber against stress corrosion are to limit the applied stress that it experiences or to prevent the buildup of water and hydroxyl ions at the fiber surface. Moisture may permeate through a polymer coating while it is being stored under tension, as found in material wound on drums, or during deployment on hydrous terrain.
Determining the chemical and physical effects of moisture vapor permeability on the optical fiber requires balancing several conflicting factors, such as effects of polar groups, fluorinated substituents, symmetry and segmental mobility. Polar groups that improve adhesion to glass and prevent buildup of water at the interface also increase moisture vapor permeability. This is due to the strong localized interaction between the small polar water molecule and polar groups within the polymer. Both low moisture vapor permeability and good adhesion are important factors in preventing stress corrosion of the optical fiber.
The process of permeation of water through a polymer film involves wetting of the polymer surface, sorption of water into the polymer matrix, diffusion through the film along a concentration gradient and desorption of water from the polymer surface. Incorporation of fluorine into the polymer chain reduces surface wettability and sorption of water. Halogenated polymers have the lowest moisture vapor permeabilities of all polymers.
Coating optical fibers in-line with a polymer resin requires resolution of certain problems that may arise in the formulation of the resin and in the direct coating of the resin onto the glass fiber. Problems which may arise in formulation of the resin are lumps, high viscosity and low polymerization rate. To avoid these problems,
1) the initiator must be thoroughly dissolved into the resin in such a manner to avoid lumps, PA1 2) the viscosity of the resin must be adjusted to within a desirable range PA1 3) an optimum amount of photoinitiator must be employed since too high a photoinitiator concentration can cause a lower polymerization rate because of the generation of an uneven distribution of free radicals and PA1 4) precautions must be taken to prevent oxygen inhibition. PA1 1) the critical surface energy of the solid glass core (.gamma..sub.s) must be greater than the critical surface energy of the liquid prepolymer resin (.gamma..sub.1) for a smooth and continuous coating, PA1 2) film thickness must be properly adjusted because too thin a coating results in reduced scratch and solvent resistance, whereas a coating that is too thick will result in an undercure at the coating/substrate interface.
Application of the coating in-line with the drawing of the glass fiber requires low viscosities (10.sup.3 -10.sup.4 centipoises), rapid solidification and fast cross-linking reactions. Ultraviolet (UV) cures are preferable but rapid thermal cures are possible.
Problems that may arise during in-line coating and curing of the optical fiber include poor wetting of the substrate, rapid shrinkage of the resin resulting in the creation of internal stresses in the coating material and incomplete overall film curing as evidenced by a tacky surface. To avoid these problems,
Hermetic coatings have been developed which require chemical vapor deposition. These hermetic coatings of silicone oxynitride, silicon carbide and titanium carbide do not allow any moisture to permeate and thus provide protection against stress corrosion and resist to attack by acid and alkali. However, the application of a hermetic coating results in a loss in dynamic fatigue strength as compared to the strength of an uncoated fiber.
A polymer coating having a low index of refraction (less than that of the silica glass core, n.sub.r =1.458, or other optical fiber) may be used as a cladding. As taught in U.S. Pat. No. 4,511,209, a fluoropolymer coating/cladding system has been developed by Ensign-Bickford utilizing aliphatic fluoropolymers, which are linear polymers. These fluoropolymer coatings have a water permeability and water vapor transmission rate of about an order of magnitude better than comparable commercial polymer coatings. The coating are not hermetic but protect against static fatigue and adhere to the glass surface.
Infinite network polymers have some advantages over linear polymers in that they are stronger as defined by tensile modulus, have no melting points but thermally decompose, have little or no solubility in solvents, have less defined glass transition temperatures and as thermosetting, not thermoplastic, polymers are capable of a UV cure. It is believed that there has been no reported use of an infinite network polymer containing fluorine used as a coating or cladding for an optical fiber.